Since having excellent energy conversion efficiency, a fuel cell such as a polymer electrolyte fuel cell (PEFC) has attracted attention as a next-generation energy source.
For example, a polymer electrolyte fuel cell includes a unit cell structure 9 as shown in FIG. 1. As shown in FIG. 1, in the unit cell structure 9, a positive electrode 92 and a negative electrode 93 are laminated so that a solid polymer electrolyte membrane 91 is interposed between the electrodes, and a pair of bipolar plates 94 and 95 is laminated so that the laminated body including the positive electrode 92, the solid polymer electrolyte membrane 91, and the negative electrode 93 is interposed between the bipolar plates. Grooves 96 used as oxygen gas (air) supplying passages and grooves 97 used as hydrogen gas supplying passages are formed on the bipolar plates 94 and 95, respectively. A fuel cell is generally formed by laminating a plurality of the unit cell structures 9.
In the unit cell structure 9, oxygen (air) is supplied to the grooves 96 of the bipolar plate 94 facing the positive electrode 92, and hydrogen is supplied to the grooves 97 of the bipolar plate 95 facing the negative electrode 93. Further, in the negative electrode 93, hydrogen is decomposed into electrons and hydrogen ions (H+). Meanwhile, in the positive electrode 92, oxygen reacts to the hydrogen ions and electrons, and water is generated. It is possible to generate an electromotive force between the electrodes 92 and 93 by the reactions in the positive and negative electrodes 92 and 93.
Metal bipolar plates having excellent impact resistance are used as the bipolar plates 94 and 95 of the fuel cell. Since also having excellent formability, as shown in FIG. 1, the metal bipolar plate is also advantageous in forming the grooves 96 and 97 on the bipolar plates 94 and 95. Further, in terms of the prevention of the increase of the internal resistance of the fuel cell, there is a demand for the bipolar plate to have small contact resistance and excellent electron conductance.
Furthermore, the bipolar plates 94 and 95 of the fuel cell are exposed to the hydrogen ion generated in the negative electrode 93 or oxygen supplied to the positive electrode 92. If the bipolar plates 94 and 95 are corroded by the hydrogen ions or dissolved oxygen, there is a danger that the internal resistance of the fuel cell is increased because the electron conductance of the bipolar plate decreases. For this reason, the bipolar plates 94 and 95 are required to have excellent corrosion resistance against the hydrogen ions or oxygen.
For example, the following bipolar plate has been developed as the metal bipolar plate for the fuel cell.
That is, for example, there has been a bipolar plate, which is composed of a thin plate made of titanium, a titanium alloy, or stainless steel on which noble metal is plated (see Patent Document 1).
Further, there has been a bipolar plate including a metal material layer that is made of Ti or the like, and a protective layer that covers the metal layer and is made of Ti nitride or Ti-alloy nitride (see Patent Document 2).
Furthermore, there has been a titanium-based material for a bipolar plate that is manufactured by adding boron origin such as AIB12 during the manufacture of a titanium substrate and precipitating TiB boride particles (see Patent Document 3).
However, the bipolar plate disclosed in Patent Document 1 has excellent corrosion resistance and stability, but uses rare noble metal. Therefore, there has been a problem in that the manufacturing cost of the bipolar plate is significantly high. Further, if the amount of noble metal to be used is decreased to reduce the cost, plating adhesion deteriorates. For this reason, there has been a danger that the plated noble metal film is peeled. Furthermore, when a plated film is formed on a thin plate (substrate) made of, for example, Al or SUS (stainless steel), a local cell is formed at a pinhole of the plated layer. For this reason, there has been a danger that local corrosion such as pitting corrosion is generated on the substrate.
In addition, since having insufficient corrosion resistance, the bipolar plate disclosed in Patent Document 2 is relatively easily corroded by hydrogen ions and the like. For this reason, there has been a danger that contact resistance is increased.
Further, the titanium-based material for a bipolar plate, which is disclosed in Patent Document 3, is composed of a substrate where TiB-based boride is dispersed, but the substrate has problems in terms of ductility and formability. For this reason, it is difficult to manufacture a thin plate, or to shape the bipolar plate. Furthermore, during the forming or the operation of the fuel cell, the TiB-based boride is apt to be separated from the substrate, and there is a danger that corrosion occurs from the separated portion. In addition, the amount of boride may be decreased in order to improve the ductility and formability. However, in this case, the boride exposed to the surface is decreased, and there is a danger that a contact area between a portion having electron conductance and other members is decreased. As a result, there is a danger that contact resistance is increased.    [Patent Document 1] JP2000-123850 Unexamined Patent Publication (Kokai)    [Patent Document 2] JP2000-353531 Unexamined Patent Publication (Kokai)    [Patent Document 3] JP2004-273370 Unexamined Patent Publication (Kokai)